Advanced usage: example with Doctrine

That's not really an advanced topic, but the sample will be a bit hard to read (as it takes many files),
so let's say it targets more experienced Symfony users.

In this demo, we're displaying a MyArray of MyElements, both persisted into the database. The MyArray entity
only have an id and a name, and MyElement has id, MyArray's id and a value. This is quite the same as we seen
in all demos, but persisted.